1. Field of the Invention
The present invention relates to IGBTs, and more particularly to an IGBT capable of achieving increased flexibility in setting a threshold voltage while achieving suppressed reduction in channel mobility.
2. Description of the Background Art
In recent years, the use of silicon carbide as a semiconductor material for a semiconductor device has been studied from the viewpoint of a higher breakdown voltage, loss reduction, the use in a high-temperature environment and the like of the semiconductor device.
Silicon carbide is a wide band gap semiconductor having a wider band gap than that of silicon which has been conventionally and widely used as a semiconductor material for a semiconductor device. By using silicon carbide as a material for a semiconductor device, therefore, a higher breakdown voltage, on-resistance reduction and the like of the semiconductor device can be achieved.
A semiconductor device including silicon carbide as a semiconductor material also has the advantage of exhibiting less performance degradation when used in a high-temperature environment than a semiconductor device including silicon as a semiconductor material.
Of such semiconductor devices including silicon carbide as a semiconductor material, semiconductor devices such as MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors), in which the presence or absence of formation of an inversion layer in a channel region is controlled with a prescribed threshold voltage as a boundary to conduct or interrupt a current, have been studied from various angles including adjustment of a threshold voltage and improvement in channel mobility (see Non-Patent Literature 1 (Sei-Hyung Ryu et al., “Critical issues for MOS Based Power Devices in 4H—SiC,” Materials Science Forum, 2009, Vols. 615-617, pp. 743-748), for example).
In an N channel IGBT, for example, a p body region of a p conductivity type is formed, and a channel region is formed in the p body region. By increasing the concentration (doping concentration) of a p type impurity (B (boron), Al (aluminum), for example) in the p body region, a threshold voltage can be shifted positive, and the IGBT can be brought closer to a normally off type or made as a normally off type.
In a P channel IGBT, contrary to the N channel IGBT, by increasing the concentration of an n type impurity in an n body region, a threshold voltage can be shifted negative, and the IGBT can be brought closer to a normally off type or made as a normally off type.
Unfortunately, when the threshold voltage is adjusted by increasing the p type impurity concentration in the p body region or the n type impurity concentration in the n body region, channel mobility decreases significantly.
The reason for the significant decrease in channel mobility is that the increase in p type impurity concentration or n type impurity concentration causes noticeable scattering of channel electrons, such as scattering of electrons due to the p type impurity or the n type impurity or scattering of electrons trapped in an interface.
For this reason, the p type impurity concentration in the p body region is set to about 1×1016 cm−3 to 4×1016 cm−3, for example. Consequently, it has been difficult to flexibly set a threshold voltage while ensuring sufficient channel mobility in a conventional IGBT, particularly to bring the IGBT closer to a normally off type or make the IGBT as a normally off type to a sufficient degree.